Water treatment-lecture-2-eenv

1. Islamic University of Gaza ‐Environmental Engineering Department
Water TreatmentWater Treatment
EENV 4331
Lecture 2: Aeration
Dr. Fahid Rabah
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2. 2.1 Purpose of Aeration in water Treatment
Aeration is used for many purposes in water Treatment theAeration is used for many purposes in water Treatment , the
main uses are:
1. Increase the dissolved oxygen concentration in water.1. Increase the dissolved oxygen concentration in water.
2. Decrease CO2 concentration thereby reduce its
corrosiveness.corrosiveness.
3. Reduce taste and odor caused by dissolved gases such as
Hydrogen sulphide (H2S) and methane (CH4) that areHydrogen sulphide (H2S) and methane (CH4) that are
removed during aeration.
4. Oxidize iron and manganese from their soluble to4. Oxidize iron and manganese from their soluble to
insoluble states and cause them to precipitate so that they
maybe removed by sedimentation and filtration processes.
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5. Remove certain volatile organic compounds.

3. 2.2 Types of aerators used in water Treatment
The most common aerators used in water treatment are :The most common aerators used in water treatment are :
1. Gravity Aerators: In gravity aerators, water is allowed to fall
by gravity such that a large area of water is exposed toby gravity such that a large area of water is exposed to
atmosphere, sometimes aided by turbulence. Cascade
aerators and Multi‐tray aerators are two examples of this
type.
2. Fountain Aerators : These are also known as spray aerators
with special nozzles to produce a fine spray.
3. Injection or Diffused Aerators : It consists of a tank with
perforated pipes, tubes or diffuser plates, fixed at the
b tt t l fi i b bbl f itbottom to release fine air bubbles from compressor unit.
4. Mechanical Aerators : Mixing paddles as in flocculation are
d P ddl b ith b d t th f
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used. Paddles may be either submerged or at the surface.

4. Cascade AeratorCascade Aerator
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5. Cascade AeratorCascade Aerator
5

6. Cascade AeratorCascade Aerator
6

7. 7
Cascade Aerator

8. 8
Cascade Aerator

9. 9
Cascade Aerator

10. 10Cascade Aerator

11. Cascade Aerator
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Cascade Aerator

12. Multi Tray Aerators
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13. Multi Tray Aerators
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14. Multi Tray Aerators
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Multi Tray Aerators

15. Spray Aerators: Nozzles
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Spray Aerators: Nozzles

16. Spray Aerators: Nozzles
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Spray Aerators: Nozzles

17. Spray Aerators: Nozzles
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Spray Aerators: Nozzles

18. Diffused Air System
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19. 19
Diffused Air System

20. Mechanical AeratorsMechanical Aerators
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21. Cascade AeratorCascade Aerator
Cw,e = gas concentration in the effluent water, mg/L
Cw,0 = gas concentration in the influent water, mg/L
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,
Cs = gas saturation concentration in water at specific temperature , mg/L
h = height of one step, m

22. Cascade AeratorCascade Aerator
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23. Cascade AeratorCascade Aerator
3
10*
*)( ggas
a
V
CMW
C 
Ca = gas concentration in air, mg/L
(MW)gas = molecular weight of gas, gram
C i i i
10*airV
Cg = gas concentration in air, ppm
Vair = volume of 1 mole of air at a given pressure and temperature, L
nRT
P
nRT
Vair 
n = number of air moles, ( in this case n =1)
R = universal gas constant, 0.08285 (atm.L)/(mole.K)
T = temperature in degrees Kelven.
P = air pressure , atm
Typical values of Cg :
CH4 = 1.5‐2.00 ppp
H2S = 20‐30 ppm
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CO2 = 350‐450 ppm

24. Cascade AeratorCascade Aerator
Example:
Calculate the saturation concentration of H2S in water at air temperature of 20 0C
Solution:
(MW) 2+32 1 34 1 g
and a pressure of 1 atm if its concentration in air is 30 ppm.
(MW)gas = 2+32.1 = 34.1 g
LVair 29.24
1
)15.27320(*08285.0*1



LmgCa /042.0
10*29.24
30*1.34
3

From the Table , kD for H2S at 20 oC = 2.87
1
From the Table , kD for H2S at 20 C 2.87
Cs = KD* Ca = 2.87 * 0.042 = 0.1205 mg/L
You can calculate CS for CO2, O2, CH4 in the same procedure of this example. S 2, 2, 4 p p
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25. Cascade AeratorCascade Aerator
Example:
C d i b d h f d h d hCascade is to be used to remove methane from ground water. The cascade has
5 steps each 0.40 m. The initial concentration of methane in the ground water is
0.80 mg/L and its saturation concentration (CS ) in water under atmospheric pressure
i i l t 0 /Lis is close to 0 mg/L.
‐What is the efficiency of methane removal after the 5 steps?
‐What is the effluent methane concentration?
l iSolution:
From the table in a previous slide: kCH4 = 27 %
K = 1‐ (1‐k)n = 1‐ (1‐ 0.27)5 = 0.793
,0,, wew
CC
CC
K



80000
80.0
793.0 ,


 ewC
Cw,e = (CCH4) effluent = 0.166 mg CH4/L.
0,ws CC 80.00.0
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26. Cascade AeratorCascade Aerator
Example:
h i h l h f h C d i h i l if i iWhat is the length of the Cascade aerator in the previous example if its capacity
is 35 m3/h per 1 m length of the step. The total water flow to be treated is 625 m3/h.
What is the total surface area of the cascade if the width to height ratio of the steps
i 1 1?is 1:1?
Solution:
 L cascade = 625/35 = 17.86 m
 Acascade = (number of steps ‐1)*step width*Lcascade
= (5‐1)*0 40*17 86 = 28 58 m2 (5 1) 0.40 17.86 28.58 m
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27. Multiple tray aerator
This is an economical solution that occupies little space.
It consists of three to eight trays with perforated bottoms. The
p y
holes are 5 to 13 mm in diameter and spaced at 75 mm center to
center.
 The trays are placed one on top of another with a 30‐50cm space e ays a e p aced o e o op o a o e a 30 50c space
between them.
 The water is evenly distributed in the first tray through porous
pipes or by stopping the gush with a bar (as shown in the figure) sopipes, or by stopping the gush with a bar (as shown in the figure), so
that the water trickles into the tray at a rate of 40‐200 L/min per
square meter of the tray’s surface.
Small droplets are then sprayed into the next traySmall droplets are then sprayed into the next tray.
To obtain a finer spray, the aerator trays can be filled with thick
gravel approximately 10cm deep (50‐150 mm) in diameter.
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28. Multiple tray aeratorp y
The removal of carbon dioxide by multiple tray aerators is represented by
h f ll ( )the following equation ( AWWA,1990):
kn
e
C 
 e
C 0
C = effluent concentration, mg/l
C0 = influent concentration, mg/l
k removal rate constant from 0 28 to 0 37 for COk = removal rate constant, from 0.28 to 0.37 for CO2.
n = number of trays
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29. Multiple tray aeratorp y
Example:
A ground water containing 8 mg/L of CO2 is to be degasified using multiple trayA ground water containing 8 mg/L of CO2 is to be degasified using multiple tray
aerator with three steps. The groundwater flow rate is 990 L/min. The k value
is 0.33 and the hydraulic loading rate of the trays is 400 L/(min.m2)
1 What is the CO concentration in the treated effluent?1. What is the CO2 concentration in the treated effluent?
2. What is the size of the trays if the length to width ratio is 2:1?
Solution:
1. C = C0 * e –kn = C0 * e –0.33X3 = 8*0.3716 = 2.98 mg CO2/L
2. Area of each tray = (990 L/min)/[(400L/(min.m2)] = 2.47 m22. Area of each tray = (990 L/min)/[(400L/(min.m )] = 2.47 m
Dimensions of the tray : L = 2 W
L* W = 2.47 m2
2W*W = 2.47 m2
W = 1.57 m
L = 3 14 m
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L = 3.14 m

30. Removal of Iron and Manganese from water by Aeration:Removal of Iron and Manganese from water by Aeration:
 Iron and manganese in excess of 0.30 mg/L and 0.05 mg/L , respectively ,should be
removed according to secondary standards of EPA. They cause staining of laundry and
plumbing fixtures, produce unacceptable taste and color.
Aeration provides the dissolved oxygen needed to convert the iron and manganese
from their ferrous and manganous forms to their insoluble oxidized ferric and manganic
forms. The produced precipitates of ferric hydroxide and manganic oxide are then
d b di t ti f ll d b id d filt tiremoved by sedimentation followed by rapid sand filtration.
 It takes 0.14 mg/L of O2 to oxidize 1 mg/L of iron; and 0.29 mg/L of O2 to oxidize 1 mg/L
of manganeseof manganese.
Oxidation reaction of iron is:
4 Fe(HCO3)2 + 2H2O + O2 > 4 Fe(OH) 3 ↓ + 8CO23 2 2 2 3 2
Oxidation reaction of iron is:
2MnSO4 + 2H2O + O2 > 2MnO2 ↓ + 2H2SO4
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2Mn(HCO3)2 + O2 > 2MnO2 ↓ + 2H2O + 4CO2